The proposed research is aimed at achieving a better understanding of the specific cellular mechanisms by which various organic and inorganic ions are transported across the basolateral (sinusoidal and lateral) and apical (canalicular) membranes of the hepatocyte. Studies are aimed at both the identification of specific transport processes and their underlying regulatory mechanisms. Such knowledge will hopefully lead to further insights into the function of the liver both in health and during bile secretory failure (cholestasis). The proposal is divided into four broad areas of investigation. The first deals with the role of anion exchange mechanisms in hepatic organic anion transport. Studies of bile acid, sulfobromophthalein, folate and methotrexate transport will be performed using basolateral and canalicular liver plasma membrane (LPM) vesicles. The second area of investigation involves the role of membrane phosphorylation in the regulation of hepatic ion transport. Phosphorylation of basolateral and canalicular membranes induced by cyclic adenosine monophosphate, Ca++/calmodulin, epidermal growth factor and tumor-promoting phorbol esters will be quantitated and specific phosphoproteins identified by gel electrophoresis. In addition, following in vitro phosphorylation, effects on various transporters will be studied. Third, studies of calcium transport will be performed in highly-purified basolateral and canalicular LPM vesicles. Specifically, both Na+/Ca++ exchange mechanisms and ATP-dependent Ca++ transport will be studied. Finally, the effects of albumin on the uptake of unconjugated bile acids and BSP by basolateral LPM vesicles will be studied.